An expandable artists' frame of the kind is disclosed in IT-A-1,031,812 and in the corresponding U.S. Pat. No. 4,050,498.
In this frame the ends of the frame sides are cut at 45.degree. and slidably housed into hollow angular supports, whereby a V-shaped strip-like spring is also housed in the bisecting line of each hollow angular support between the abutting bevelled ends of two adjacent frame sides and tends to bias apart the side ends from each other.
At the ends of said frame sides are also provided longitudinal slots which allow said hollow angular supports to be slidably inserted along said slotted side ends.
Besides a hollow box-like construction, the known angular or L-shaped supports have a number of structural and functional expedients. These include a bisector-like diagonal wall defining inside the hollow support, between the abutting side ends, an air space capable to house a preloading V-shaped spring, and outside the hollow support, a corner filling wall portion capable of supporting the canvas in the corner area, when the bevelled side ends are at a distance from each other, that encompasses all the possible different expansion positions of the artists' frame.
The frames according to the teaching of U.S. Pat. No. 4,050,498 provide a functionally acceptable preloading action onto the canvas fixed to the artists' frame or stretcher. However, the preloading action allows only a limited expansion stroke of the frame sides.
Furthermore, from a manufacturing and handling point of view, the frames present limits and shortcomings.
First of all the artists' frames require four plastics hollow angular supports. The structural and functional expedients of the disclosed hollow angular supports increase the synthetic material costs and require complicated and costly moulds.
Notwithstanding these expedients, the stiffness obtained with the hollow angular supports is limited by an opening at the internal corner of the hollow angular support. This opening is necessary for inserting the respective V-shaped preloading spring.
Furthermore, in order to permit insertion of the spring, the preloading springs require a preliminary closure of the spring legs, practically with an abutment of the legs one against the other, and consequently a strong closing force which can only be applied by means of special tools, so that the V-shaped springs are not suitable for manual introduction by the user.
It is further to be noted, that the preloading force exerted by these known sprigs decreases, that it is determined by the spring dimensions, and that it is not adjustable by the user.
It is further to be pointed out, that the components of these artists' frames, except the canvas and the staples or nails, that is the frame sides, the hollow angular supports and the springs are dimensionally mutually adapted for forming the frame in question, and do not allow the use of single components per se, that is as distinct parts for making, for example, expandable frames of other kinds or sizes. In other words, the single components of the artists' frame disclosed in U.S. Pat. No. 4,050,498 can not be used with expandable frames in general, for example with expandable frames having wedges, which as is known require the machining of appropriate seats or slots for housing the wedges into the frame sides at the respective internal corner.
The arrangement of these known wedges for statically preloading the canvas and the respective seats or slots for housing the wedges can be seen, from example, in U.S. Pat. No. 3,882,616. However, the artists' frame disclosed in this patent is not provided with springs for dynamically preloading the canvas.
The present invention further relates to improved hollow angular supports for expandable frames in accordance with the present invention.
A further expandable frame is disclosed in DE-C-581,549. In this artists' frame, in the respective bevelled opposite ends of adjacent frame sides are provided two opposite cylindrical seats. One seat houses a guiding pin and the other seat houses a compressible helicoidal spring which, in its rest position, holds the abutting side ends apart from each other. In the side ends are further provided two hollow seats for housing the ends of a securing bracket spring, which while acting against the helicoidal spring, holds two respective abutting side ends resting one on top of the other for fixing the canvas onto the frame. After having fixed the canvas, the bracket springs are removed and the helicoidal springs preload the canvas. This artists' frame requires expensive machining operations, a considerable number of components, and the springs do not allow a preloading adjustment.
Furthermore, the frame sides are connected together only by the pins and helicoidal springs so that the frame practically does not have any stiffness, which by contrast is strongly required in artists' frames or stretchers. An expandable artists' frame having corner springs is disclosed in FE-A-2,439,683.
In this frame the frame sides are slidably embraced at their ends from the interior by L-shaped supports having legs with a U-shaped cross section and which are provided at the internal corner of the L-shaped with a quadrangular section. Parallel to the U-shaped support legs is provided a respective helicoidal spring which is inserted onto a screw. The screw is screwed at one end thereof into the quadrangular section, and at the other end is fixed to a bracket, which in turn is fixed to the respectively associated frame side by means of two screws. A nut screwed onto the screw compresses the helicoidal spring, whereby the associated frame side is biased apart from the abutment area of adjacent sides so that a consequent preloading action of the canvas is reached.
This frame requires a large number of components for each frame corner, and the L-shaped supports as well as the screws housing the springs consist of several parts which are welded or riveted together.
The screws holding the springs are in turn fixed to the associated frame sides by two screws. For each frame side thickness, it is necessary to provide L-shaped supports having a corresponding cross section size of the U-shaped legs. The preloading action is indeed adjustable between a minimum and a maximum value by compressing the helicoidal springs. However, a subsequent preloading is not possible. The L-shaped supports cannot cover the air space between two adjacent opposite side ends so that under a strong canvas shrinkage this frame can undergo twists.